1. Field of the Invention
The present invention relates generally to a ferroelectric liquid crystal apparatus having a display portion formed by a pair of display areas and to a method of driving such a ferroelectric liquid crystal apparatus. More particularly, the present invention relates to a ferroelectric liquid crystal apparatus and driving method in which a predetermined voltage signal is applied to one of the separate display areas during partial rewriting (i.e., partial scanning) of the contents displayed in the other display area for partial scanning of the device so as to reduce the difference in contrast between the display areas. Thereby, improved image display quality is provided.
2. Description of the Related Art
Recently, ferroelectric and TN-type liquid crystal apparatus have been intensively developed for use as image displays. Ferroelectric liquid crystal apparatus are especially advantageous since they provide a memory-like capability, a fast its response speed, and contrast is independent of the duty ratio. Accordingly, it would be desirable to apply the ferroelectric liquid crystal apparatus, for instance, to large-capacity dot matrix apparatus.
FIG. 2 is a schematic view showing one example of the above-described ferroelectric liquid crystal apparatus. As illustrated, this example is provided with a group 1 of scanning electrodes, a group 2 of upper signal electrodes (as viewed in the figure) and a group 3 of lower signal electrodes (as viewed in the figure). The group 1 of scanning electrodes is oppositely spaced apart from groups 2 and 3 of upper and lower signal electrodes with a ferroelectric liquid crystal interposed therebetween. A scanning electrode driving circuit 4 is disposed to apply a voltage to the group 1 of scanning electrodes, while an upper signal electrode driving circuit 5 and a lower signal electrode driving circuit 6 are disposed to apply voltages to the group 2 of upper signal electrodes and the group 3 of lower signal electrodes, respectively. The group 1 of scanning electrodes and the group 2 of upper signal electrodes cross each other to form an upper display area 7, while the group 1 and the group 3 of lower signal electrodes cross each other to form a lower display area 8. In each of the upper display area 7 and the lower display area 8, picture elements are arranged in a matrix form.
FIG. 3 is a schematic diagram showing exemplary forms of applied voltages which act to drive the above-described matrix type display apparatus on a multiplexing basis. As illustrated, scanning pulses 9 are sequentially applied to the group 1 of scanning electrodes, while information signal pulses 10 are sequentially applied in parallel to group 2 or 3 of signal electrodes in synchrony therewith so as to provide a visual image display. If an electric field is not subsequently applied, the last image which was displayed is held. Therefore, the image or information which is required for display on the display apparatus is only rewritten when the necessity for global rewriting arises or when a particular portion (display area) is rewritten.
Although the present inventors carried out the above-described conventional driving method, they found that image contrast markedly differs during a partial-scanning period than during a non-scanning period. This is noticeable, for instance, in the case of the panel-shaped display apparatus of the horizontal split type shown in FIG. 2, which is made of upper display area 7 and lower display area 8 which are separately driven by the groups 2 and 3 of upper and lower signal electrodes. That is, when the upper display area 7 is being scanned in a partial-scanning mode, the contrast of an image displayed in the lower display area 8 (which is not being scanned) is noticeably different from an image displayed in the upper display area 7. This phenomenon, of course, seriously affects the quality of the displayed image. The inventors believe that the difference in image contrast between a scanning period and a non-scanning period arises because, although when no electric field is being applied to a ferroelectric liquid crystal apparatus, the molecules thereof remain oriented in one of two stable states so that the molecules assume the other state if a threshold-exceeding electric field is applied upon the application of an electric field not exceeding the threshold, the molecules nevertheless change their positions more or less, although their state does not substantially change. In FIG. 4, this phenomenon is shown plotted as the relationship between time and a variation in the quantity of light transmitted under cross-nicol conditions as an applied voltage 11 not exceeding the threshold is applied over time. Reference numeral 12 denotes a curve which indicates the corresponding variation in the quantity of transmitted light. It is seen in FIG. 4 that the average quantity of light transmitted during the application of a voltage differs from the quality of light transmitted during application of no voltage. Accordingly, as shown in FIG. 3, during a scanning period, a non-selection voltage (not exceeding the threshold) shown in FIG. 4 is applied to picture elements on non-selected scanning electrodes so that the quantity of light which is transmitted during the scanning period is different than the quantity of light which is transmitted during the non-scanning period. As a result, contrast varies between the scanning and non-scanning periods, as well as between a portion which is being scanned and a portion which is not being scanned. If variation in contrast over time is to be suppressed to a minimum, a refresh scanning (driving) method may be adopted in which information signal pulses are always applied to the signal electrodes so that the contrast is kept constant.